Multipath reception conditions give rise to ghosts in NTSC television reception. Multipath signals that arrive at the receiver with enough time displacement from the principal signal as to cause discernible ghosts in a received television image are referred to as "macro-ghosts". Multipath signals which arrive over a transmission path of lesser length than the strongest or "principal" signal reach the receiver earlier and are referred to as "pre-ghosts"; the ghost images they cause in a received television image appear to the left of the desired image. Pre-ghosts occurring in off-the-air reception can be displaced as much as 6 microseconds from the "principal" signal, but typically displacements are no more than 2 microseconds. Multipath signals which arrive over a transmission path of greater length than the strongest or "principal" signal reach the receiver later and are referred to as "post-ghosts"; the ghost images they cause in a received TV image appear to the right of the desired image. Typically, the range for post-ghosts extends to 40 microseconds displacement from the "principal" signal, with 70% or so of post-ghosts occurring in a sub-range that extends to 10 microseconds displacement. Multipath signals that arrive at the receiver with not enough time displacement from the principal signal as to cause discernible ghosts in a received television image, but which affect transient response, are referred to as "micro-ghosts". Macro-ghosts are more common in over-the-air terrestrial broadcasts than cablecasting, in which micro-ghosts commonly occur because of reflections. Similar multipath reception conditions obtain in digital television (DTV) systems as in NTSC and other analog television systems.
In September 1995 the Advanced Television Systems Committee (ATSC) published a standard for digital high-defmition television (HDTV) signals that has been accepted as the defacto standard for terrestrial broadcasting of digital television (DTV) signals in the United States of America. The standard will accommodate the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal. The standard uses vestigial-sideband (VSB) amplitude modulation (AM) to transmit the DTV signals, designed for transmission through 6-Mz-bandwidth ultra-high-frequency (UBF) channels that correspond to channels currently used for analog television transmission.
DTV transmitted by VSB AM during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments or data lines. Each segment of data is preceded by a data segment synchronization (DSS) code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The segments of data are each of 77.3 microsecond duration, and there are 832 symbols per data segment for a symbol rate of about 10.76 million bauds or symbols per second. The initial line of each data field is a data field synchronization (DFS) code group that codes a training signal for channel-equalization and multipath suppression procedures. The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting proposed in the ATSC standard. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without preceding.
The carrier frequency of a VSB DTV signal is 310 kHz above the lower limit frequency of the TV channel. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
Ghosts are a problem in digital television (DTV) transmissions as well as in NTSC analog television transmissions, although the ghosts are not seen as such by the viewer of the image televised by DTV. Instead, the ghosts cause errors in the data-slicing procedures used to convert symbol coding to binary code groups. If these errors are too frequent in nature, the error correction capabilities of the DTV receiver are overwhelmed, and there is catastrophic failure in the television image. If such catastrophic failure occurs infrequently, it can be masked to some extent by freezing the last transmitted good TV images, such masking being less satisfactory if the TV images contain considerable motion content. The catastrophic failure in the television image is accompanied by loss of sound.
The training signal or ghost-cancellation reference (GCR) signal in the initial line of each data field of an ATSC-standard DTV signal is a 511-sample pseudo-random noise sequence (or "PN sequence") followed by three 63-sample PN sequences. A 511-sample PN sequence is referred to as a "PN511 sequence" and a 63-sample PN sequence is referred to as a "PN63 sequence". The middle ones of the 63-sample PN sequences in the field synchronization codes are transmitted in accordance with a first logic convention in the first line of each odd-nunbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. This training signal has not worked well in practice, however.
The middle PN63 sequence of the ATSC field synchronization code, as separated by differentially combining corresponding samples of successive field synchronization code sequences, can used as a basis for detecting ghosts. Pre-ghosts of up to 53.701 microseconds (4+511+63=578 symbol epochs) before the separated middle PN63 sequence can be detected in a discrete Fourier transform (DFT) procedure without have to discriminate against data in the last data segment of the preceding data field. However, the post-ghosts of such data can extend up to forty microseconds into the first data segments and add to the background clutter that has to be discriminated against when detecting pre-ghosts of the separated middle PN63 sequence. Post-ghosts of up to 17.746 microseconds (63+104+24=191 symbol epochs) after the separated middle PN63 sequence can be detected in a discrete Fourier transform (DFT) procedure without have to discriminate against data in the A precode and in the data segment of the succeeding data field. Longer-delayed post-ghosts have to be detected while discriminating against background clutter that includes data. A match filter for the PN63 sequence may not have enough peak energy in its response that detection of longer-delayed post-ghosts is sufficiently sensitive. The middle PN63 sequence of the ATSC field synchronization code provides more pre-ghost canceling capability than required in practice, but insufficient post-ghost canceling capability. Post-ghosts delayed up to forty microseconds after principal signal occur in actual practice. However, pre-ghosts preceding the principal signal by more than four microseconds are rare, according to page 3 of the T3S5 Report Ghost Canceling Reference Signals published Mar. 20, 1992 by the ATSC.
If one seeks to exploit the autocorrelation properties of the PN511 sequence in the ATSC DTV signal for selection of ghosts in a DFT procedure, the selection filter has to discriminate PN511 sequence and its ghosts from background clutter that includes data and the initial and final PN63 sequences. This background clutter has substantial energy, so weaker ghosts of the PN511 sequence are difficult to detect.
The higher energy response of the PN511 autocorrelation filter used for ghost detection cannot be fially exploited because data and the initial and final PN63 sequences increase so much the energy of the background clutter that the filter is to discriminate against.
The objective of the inventor was to find a way to characterize the transmission channel through which a DTV signal was received that would be better in terms of speed and in terms of increased capability for suppressing long-delayed post ghosts. At the time the invention was made it was known that long-term accumulation procedures could be employed together with a matched filter algorithm to derive a better training signal from the PN sequences in the DFS signals. Accumulation is done on corresponding samples of the DFS signal in a comb filtering procedure extending over several data fields. So, the time required in order to extract such a better training signal extends over several data field intervals. When multipath conditions are dynamic in nature, the long-term accumulation procedures repeatedly fail to open the data eyes sufficiently to permit decision feedback methods to take over adjustment of channel-equalization and ghost-cancellation filter coefficients. The common wisdom at the time of the invention was that accumulation of match filter response to the longer-duration PN511 sequence would result in better separation of that signal and its ghosts from background clutter than would accumulation of match filter response to the shorter-duration PN63 sequence.
The inventor found this would be so only for accumulation over a few data fields, supposing similar background clutter accompany each signal offered for accumulation. As the number of samples being combined in the correlation procedures associated with accumulation of respective match filter responses becomes large enough for statistics to assert themselves in regard to randomness of data contributing to background clutter, the ratio of the energy of the accumulated correlated PN sequence to the energy of the accumulated uncorrelated random data and noise in the background clutter approaches the same asymptotic value for both correlation procedures. The inventor discerned that match filtering selected over random background clutter based on the total number of samples that were combined, rather than just the kernel width of the match filter.
The match filter for PN511 sequence has pronounced advantage over a match filter for PN63 sequence, the inventor understood, in rejection of response to misphasing of the kernel to a signal that cyclically repeats that kernel. The PN sequences are purposely designed to have negligible correlation with their mis-phased selves. So superior selectivity of the match filter for wider-kernel PN sequence is evidenced more during autocorrelation than when discriminating against randomly varying background clutter.
It occurred to the inventor that over a data field the energy in the accumulated match filter response to data segment synchronizing (DSS) signal is substantially twice the energy in the accumulated match filter response to the repeating PN sequences in the data field synchronizing (DFS) signal in the initial data segment of each data field. While a match filter for the DSS signal has only four symbol epochs therein, as opposed to the 637 symbol epochs that repeat themselves in the DFS signal, the DSS signal appears in each of the 313 data segments of each data field rather than once, for 4.times.313 /637=1.965 times as much energy per data field. The inventor was led by this observation to believe that match filter response to the baseband DTV signal as accumulated in a transversal filtering procedure across a plurality of data segments would generate a superior training signal. The equivalent filtering procedure of transversal filtering across a plurality of data segments, to supply input signal to a match filter for the DSS signal would be an alternative way to generate that superior training signal.